This invention relates to a hydraulic drive system for driving, for example, a large size hydraulic shovel, with the hydraulic drive system comprising at least one hydraulic actuator, a plurality of hydraulic pumps connected in parallel with one another and each connected to the hydraulic actuator to form a closed circuit therewith, and a plurality of prime movers each for driving the plurality of hydraulic pumps.
In the field of civil engineering machinery, such as, for example hydraulic shovels, a hydraulic drive system is known which comprises a single variable displacement hydraulic pump connected to one or a plurality of hydraulic actuators to form a closed circuit therewith. In this hydraulic drive system, operation of the actuators is controlled by controlling the direction in which the pump delivers pressure fluid and the flow rate of the delivered pressure fluid without using a directional control valve mounted between the hydraulic pump and the actuators. In recent years, to cope with an increase in the size of hydraulic shovels and an increase in the size of the hydraulic actuators, a hydraulic drive system has been proposed which comprises two variable displacement hydraulic pumps connected in parallel with each other and each connected to hydraulic actuators to form a closed circuit therewith. The two hydraulic pumps are driven by separate prime movers, and operation of the actuators is controlled by controlling the direction in which the two hydraulic pumps deliver the pressure fluid and the flow rate of the delivered pressure fluid by the pumps with a common operation signal.
In the hydraulic drive system comprising two hydraulic pumps and two prime movers, the pressure fluid delivered by the two hydraulic pumps can be used in good condition for operating the actuator so long as the two prime movers normally operate. However, when one of the prime movers is shut down for some reason, there is the risk that the pressure fluid delivered by the hydraulic pump connected to the normally operating prime mover is supplied to the other hydraulic pump and might cause same to rotate in the reverse direction, or the hydraulic pump might perform the function of a motor to thereby cause the prime mover connected to the hydraulic pump and other pumps connected to the prime mover, such as, lubricant pump, pilot pump and hydraulic pump for other hydraulic circuit, etc. to rotate in the reverse direction, thereby causing damage thereto.